194 MINNESOTA STATE HORTICULTURAL SOCIETY. 
ingly retarded. In the proterandrous form, the stamens received 
the benefit and the pistil was retarded. Through successive genera- 
tions, the influence of the law of balancement has been at work, the 
evolution towards a separation of the sexual organs has fairly start- 
ed, and we have the foundation in the proterandrous and proterogyn- 
ous forms for the pistillate and staminate forms ofa future dicecious 
species. Simultaneously with the development of the pistil and re- 
tardation of the stamens and vice versa, came the further adapta- 
tion of difference in time of maturity of the reproductive elements, 
with an additional protection against self-fertilization. 
The development of the reproductive organs, aided by the law 
of balancement, continued, and we have developed the heterostyled 
group. What was before accomplished to quite an extent by purely 
mechanical adaptations, is now accomplished by a differentiation 
in the reproductive elements. So greata differentiation in the re- 
productive organs surely caused a differentiation in the sexual ele- 
ments. 
Our species has now become divided against itself. The dif- 
ferentiation ofthe reproductive elements was followed by still further 
development and retardation of the reproductive organs—and we 
have nearly reached the bisexual stage, not only in morphology 
butin function. Wenow have only astep further to the complete 
separation of the sexual organs. 
In the earlier stages of the dichogamous group, self-fertiliza- 
tion was possible but mainly prevented by mechanical adaptations 
to insure cross-fertilization. In the heterostyled group self-fertil- 
ization is prevented by the great differentiation in the reproductive 
elements, and the sexual affinity destroyed. There can be no doubt 
that the differentiation into species was accomplished by variation 
of form, foliage, cell structure, etc., without a corresponding differ- 
entiation of the sexual elements. We know this from the fact that 
distinct species will sometimes cross, but not freely, in a reciprocal 
direction. I have myself, within the past ten years, produced hy- 
brids between P. angustifolia and P. Americana; between P. do- 
mestica and P. Americana; between P. Besseyi, Bailey (P. pum- 
ila, Lin.) and P. hortulana, B.; between Cerasus avium rar. and 
P. Besseyi, B. I made several hundred crosses to produce hybrids 
between our sand cherry (P. Bessey1) and horticultural varieties 
of Cerasus avium. Pollen of C. avium var.on P. Besseyi invar- 
iably proved sterile; reciprocal crosses set fruit, but they failed to 
germinate, the seed containing only a trace of the aborted ovule. 
When I finally used the pollen of a proterandrous form of P. Bess 
ona short-styled form of C. avium fertilization was effected and 
developed a normal fruit, the seed of which germinated and pro- 
duced an undoubted hybrid. The reciprocal crosses of the same 
varieties failed to fertilize a single ovule out of over fifty crosses 
made. I had applied the same principle in the production of hy- 
brids between P. hortulana and P. Besseyi with fair success. The 
successful crosses just mentioned were made with pollen which had 
not been too greatly differentiated, on a pistil which, in accordance 
with the theory advanced for the evolution of the different forms, 
tea 
